An integrated circuit includes a deep n-type well in a p-type substrate, for example to isolate a transistor in the deep well from the substrate. A deep isolation trench isolation is formed through the deep well to laterally isolate the transistor. The deep isolation trench separates a portion of the deep well from a main part of the deep well which contains the transistor; the separated portion contains no components of the integrated circuit. The separated portion of the deep well undesirably causes electrical interference with nearby circuit components.